Method of correcting for phase error induced by a down sampling routine

ABSTRACT

A method and apparatus of correcting a data signal sampled at a first rate to a data signal displayed on a video monitor at a second rate is claimed. A data signal is received at a first rate. The data signal is separated into data windows. The minimum and maximum values and positions of data points in data windows are identified relative to a reference, and displayed on a video monitor.

RELATED APPLICATIONS

This application claims the benefit under 37 C.F.R. §119 of prior filed,co-pending Provisional Application No. 60/072,248, filed on Jan. 22,1998.

BACKGROUND OF THE INVENTION

The present invention generally relates to displaying of physiologicaldata on patient monitors. More particularly, the present inventionrelates to a method and apparatus of correcting for phase error inducedby down sampling of physiological data.

Patient physiological data, such as ECG data, is commonly displayed onpatient monitors as a waveform suitable for review by medical carepersonnel. In order for medical care personnel to correctly assess theclinical import of the displayed information, it is highly desirablethat the waveforms accurately reflect the measured physiological dataregardless of the monitor type, pixel resolution, the size of the windowin which the waveforms or other data are displayed or the speed at whichthe data is sampled. Generally, this has required some specific scalefactor to be employed for each monitor type, pixel resolution capacityor display window size being employed.

SUMMARY OF THE INVENTION

One source of waveform distortion results from symmetrically plottingnon-uniformly spaced selected data points in down-sampled data. Whendown-sampling data, features of the waveform in the original data may belost or distorted by the down-sampling process. For example, theresolution of a standard cathode ray tube (CRT) monitor is not adequatefor presenting full resolution ECG data. ECG data is typically receivedfrom the patient at a rate of about 480 Hertz (Hz)₃. The ECG data isfiltered for electrical noise and other extraneous electricalinformation, reducing the data rate to about 240 Hz and presented to theclinician using a standard scroll rate of 25 millimeters (mm) per second(sec). A scroll rate of 25 mm/sec is a standard within the health careindustry. A typical 21″ CRT monitor displays a horizontal image size ofapproximately 406.4 mm with a horizontal screen resolution of 1280pixels, or 3.15 pixels/mm. At a scroll rate of 25 mm/sec, each pixelrepresents approximately 12.7 milliseconds (ms) of data. Thus, if datais received at a 240 Hz rate, a data point is displayed on the CRT every4.167 ms. This means that 3.05 data points will map to the samehorizontal pixel location on the CRT. If the data is received at adifferent rate, e.g., 120 Hz (such as is the case with blood pressurewaveforms) 1.52 data points will map to the same horizontal pixellocation on the CRT. Plotting data as a waveform in this manner resultsin the plotting of different data points on the same pixel location. Asa result, important physical features of the waveform representing thedata may be lost.

Accordingly, the invention provides an interface for accuratelydisplaying non-uniformly spaced selected data points in down-sampleddata for a computer monitor display.

According to one aspect of the invention, a method and apparatus forsynchronously plotting non-uniformly spaced selected data points indown-sampled data is provided. A data signal is received at a first rateand separated into at least one data window. Each data window has apredetermined number of data points having respective values. At leastone of either a minimum and a maximum value of the data points areidentified, thus identifying a position of the one of the minimum andmaximum value relative to a reference. The data point is then displayedhaving the one of the minimum and maximum value at the position.

The invention also provides an apparatus having an input for receiving adata signal sampled at a first rate and means for converting the datasignal to a second data signal sampled at a second lower rate and fordisplaying the second data signal on a video monitor.

It is an advantage of the invention to provide an interface foraccurately displaying non-uniformly spaced selected data points indown-sampled data for a computer monitor display.

It is another advantage of the invention to provide a method andapparatus for synchronously plotting non-uniformly spaced selected datapoints in down-sampled data.

Other features and advantages of the invention will become apparent tothose skilled in the art upon review of the following description,claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a prior art waveform wherein non-uniformly spaceddata is plotted at fixed, regularly spaced intervals on the CRT.

FIG. 1B illustrates the same waveform produced by use of the presentinvention, plotting non-uniformly spaced data at varying intervalsaccording to the invention.

FIG. 2 is a block diagram illustrating a patient monitoring systemaccording to the invention.

FIG. 3 is a flow chart illustrating the method of the invention.

Before one embodiment of the invention is explained in detail, it shouldbe understood that the invention is not limited in its application tothe details of the apparatus, composition or concentration ofcomponents, or to the steps or acts set forth in the followingdescription. For example, the invention is capable of embodiments otherthan those adopted particularly for healthcare applications. Also, itshould be understood that the phraseology and terminology used herein isfor the purpose of description and should not be regarded as limiting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 illustrates the patient monitoring system 32 of the invention.The patient monitoring system 32 acquires and displays physiologicalpatient data. While the monitoring system 32 can be used in connectionwith monitoring any kind of physiological parameter, in the preferredembodiment, the monitoring system 32 is for monitoring a patient'selectrical cardiac activity and blood pressure. Monitoring system 32 iscoupled to the patient 36 by an array of sensors or transducers whichmay include, for example, electrodes 40 mounted on the patient's chestand arm for electrocardiogram testing. Hereinafter, the term “sensor”and “transducer” will be used synonymously, and each term will bedefined as including the subject matter of the other term.

The signals derived from the sensors are converted from analog form todigital form by an analog to digital converter (A/D) 44 and provided toa converter 60 that prepares the data for display on a display monitor52. In the embodiment in FIG. 2, the A/D 44 further includes apre-processor. The digital conversion by the A/D 44 is done at a rate of480 Hz. The pre-processor then separates and filters the 480 Hz datainto packets to be processed by the converter 60. Electrical noise andother extraneous electrical signals are filtered before the data ispresented to the converter 60. The data rate after pre-processing isabout 240 Hz. In other embodiments (not shown), if the signal collectionrate is equal to or less than the rate at which the converter processesthe data, the act of separating the data into packets by thepre-processor may be avoided.

The display monitor 52 is a conventional computer-style display monitorhaving a generally rectangular cathode ray tube (CRT). The CRT includesa plurality of pixels. The vertical location of the pixels is defined bya Y-coordinate and the horizontal location of the pixels is defined byan X-coordinate. As is known in the art, each pixel is capable of beingenergized electronically so that the pixel emits light visible to theuser of the monitoring system.

FIG. 3 is a flowchart illustrating the operation of the patientmonitoring system. The system collects physiological data (56) andpre-processes the data to a first rate, and converts the analogphysiological data to digital data using an A/D converter. The converter(60) samples the collected, pre-processed physiological data 56 at asecond rate, also called the update rate. The converter 60 may beresident in a stand alone bed-side computer system, or the converter 60may be resident in a common network computer where physiological datafrom multiple patients may be centrally processed. The converter 60generates a data signal having data points that are at approximately thesame position as the data points had relative to one another in theoriginally sampled physiological data. In the converter 60, the datasignal is typically temporarily stored (62) in a buffer until the datais processed. For ECG data, the data collection rate, or the samplerate, is about 240 Hz. For Blood Pressure (BP) data, the sample rate isabout 120 Hz. The update rate operates at a predetermined speed. For allwaveforms, including those composed of ECG and BP data, the update rateis about 60 Hz, although it is contemplated that other update rates maybe used.

The collected physiological data is separated into portions or datawindows, which are then extracted (64) from the buffer (62) to beprocessed. The number of data points comprising the data window is equalto the sample rate divided by the update rate. Thus, for ECG data, thedata window is 240 Hz/60 Hz, or four data points. This is equal to aboutone point for every 16.667 ms of data.

In order to preserve the overall shape of the waveform, data points mustbe selected that best represent the waveform. Thus, the data window isdown-sampled (66) by selecting, on the average, one data point from eachdata window. In one embodiment, the data point chosen is one of either alocal minimum or a local maximum data point. In another embodiment, twodata windows (comprising eight points) are considered together. Thisaccommodates for situations in which it is desirable to choose more thanone data point from within a single data window. If more than one datapoint is chosen within a single data window, no data points are chosenfrom the adjacent data window. Choosing more than one point in a singledata window accommodates for situations in which more than one relativeminimum or maximum data point occurs within a single data window, and norelative minimum or maximum data points occur in the adjacent datawindow. Thus, when two data windows are considered together, two of theeight data points comprising the two adjacent data windows areselected—the local minimum data point and the local maximum data point.In this manner, the overall shape of the waveform is best preserved.

In the original 240 Hz data stream, each data point is separated by auniform 4.1667 ms, and each data window of four points is separated by auniform 16.667 ms. However, as a result of down sampling, the resultingseries of data points is not uniformly spaced. Thus, using the last datapoint of a data window as a reference, a time off-set for each datapoint is calculated (70). The time off-set is the time difference (orphase error) per point that is induced by fixed space plotting. The timeoff-set (in milliseconds) is calculated using the equation:Time Off-Set_(ms)=[(R _(s) /R _(u))−i]* 1000/R _(s)

where, R_(s) is the sample rate (240 Hz), R_(u) is the update rate (60Hz), i is the index number of the selected point, and 1000/R_(s) is thetime separation between points in ms (for ECG, 1000/240 Hz=4.167 ms).The time off-sets for ECG data sampled at 240 Hz and operating at anupdate rate of 60 Hz are shown below: Data Point Selected Time Off-Set 4   0 ms 3 4.167 ms 2 8.333 ms 1  12.5 ms

A position to plot a data point is then determined (74), and plotted(78). The position to plot a data point is determined by moving from thecurrent position by an amount equivalent to the time off-set for thatdata point. For example, if data point 4 is chosen from the first datawindow, and data point 3 is chosen from the second data window, the plotposition for data point 3 is determined by moving forward the timeoff-set of four data points (16.667 ms), and then moving back by thetime off-set of one data point (i.e., from the time of Data Point 4 tothe time of Data Point 3), or 4.167 ms. Thus, data point 3 of the seconddata window is plotted 16.667 ms-4.167 ms, or 12.5 ms from data point 4of the first data window. Moving forward by 16.667 ms is based on theupdate rate of 60 Hz. Moving forward at a constant 16.667 ms allows themultiple waveforms to be updated synchronously.

After a data point is plotted, the next data window of physiologicaldata is extracted (64) from the buffer (62). The process is repeateduntil all of the data is processed.

Multiple waveform may also be displayed in any given window. Becausereal time data is being displayed, a constant, periodic update ispreferred. If the update to the displayed is not constant, a noticeablejerkiness may be apparent to the human eye. In addition, each of thewaveforms on the display will potentially have different points selectedwithin the data window. The update rate, however, is constant for allwaveforms plotted. Thus, the presentation for all of the waveforms onthe display is moved forward at a fixed rate, and the display is updatedwith all data that has occurred since the previous update.

FIG. 1A illustrates an ECG waveform 6 and a blood pressure waveform 8wherein non-uniformly spaced 60 Hz data is plotted at fixed, regularlyspaced intervals on the CRT as is done in the prior art. Plotting insuch a fashion (in effect) shifts the data points relative to oneanother resulting in distortion of the waveform's shape.

As shown in FIG. 1A, general distortion due to small, rapid variationsin the size, shape or position of observable information may occur, asindicated by the region of the waveform indicated by reference numeral10. Distortion of the QRS width may occur, as illustrated by referencenumeral 14. Aberrations of the size or position of tips of waveforms mayalso occur, as indicated by the elongated and flat tips shown byreference numeral 16. Waveform peaks may also appear tilted ordistorted, as indicated by reference numeral 20. Further, time,amplitude, frequency or phase related jitter may be present, asindicated by the change in slope of the waveform shown by referencenumeral 24.

FIG. 1B illustrates a waveform produced using the down samplingtechnique of the present invention. In FIG. 1B, non-uniform data pointsare plotted as they occur, i.e., at uneven spacing as calculated byconverter 60. As shown, the ECG data as plotted according to theinvention contains sharp QRS spikes 26 that play an important roll inthe assessment of a patient's condition.

Thus, the plotting of physiological data using the present inventionminimizes distortions and aberrations caused by down-sampling the rapidvariations inherent in physiological data, and is more reflective of thetrue waveform, providing a more accurate depiction of features such asQRS width, tips and peaks present of the waveform, and slope variationsof the waveform.

Various features of the invention are set forth in the following claims.

1-29. (canceled)
 30. A method for sampling and displaying a data signal,the method comprising: receiving a first data signal sampled at a firstrate; sampling the first data signal at a second rate to generate asecond data signal having a first data point and a second data point;calculating a first time off-set from a reference point for the firstdata point; calculating a second time off-set from a reference point forthe second data point; displaying the first data point according to thefirst time off-set; and displaying the second data point according tothe second time off-set.
 31. The method as set forth in claim 30, andfurther comprising acquiring the first data signal from a patient. 32.The method a set forth in claim 30, and further comprising acquiring thefirst data signal from a patient, the first data signal including asignal of ECG data.
 33. The method as set forth in claim 30, and furthercomprising acquiring the first data signal from a patient, the firstdata signal including a blood pressure waveform.
 34. The method as setforth in claim 30, and further comprising separating the first datasignal into at least a first data window having a first reference pointand a second data window having a second reference point; sampling thefirst data signal at a second rate to generate a second data signalhaving a first data point and a second data point, the first data pointbeing sampled from the first data window and the second data point beingsampled from the second data window; calculating a first time off-setfrom the first reference point for the first data point; and calculatinga second time off-set from the second reference point for the seconddata point.
 35. The method as set froth in claim 34, and furthercomprising sampling the first data signal at a second rate to generate asecond data signal having a first data point and a second data point,the first data point being selected from one of a maximum data point anda minimum data point sampled from the first data window, and the seconddata point being selected from one of a maximum data point and a minimumdata point sampled from the second data window.
 36. The method as setforth in claim 30, and further comprising separating the first datasignal into at least one data window; and sampling the first data signalat a second rate to generate a second data signal having a first datapoint and a second data point and a minimum data point sampled from thefirst data window, and the second data point being selected from theother of the maximum data point and the minimum data point sampled fromthe second data window.
 37. The method as set forth in claim 30, andfurther comprising separating the first data signal into at least afirst data window and a second data window; and sampling the first datasignal at a second rate to generate a second data signal having a firstdata point and a second data point, the first data point being selectedfrom one of a maximum data point and a minimum data point sampled from acombination of the first data window and the second data window, and thesecond data point being selected from the other of the maximum datapoint and the minimum data point sampled from the combination the firstdata window and the second data window.